With consideration for global environmental issues, a known vehicle, for example, an automobile is provided with an idling stop controlling function which stops an engine when the vehicle is stopped at, for example, an intersection for a short time and prevents the emission of exhaust gas and the consumption of gasoline. When the idling stop controlling function is operated, the engine is not performed. However, in such a case, because the oil pressure level of a transmission is required to be maintained, an electric pump which can be driven to supply the oil (an example of operation fluid) even when the engine is not performed is used to maintain the oil pressure level of the transmission.
Because the electric pump is disposed at an engine room, the secure operation of the electric pump at high temperature is required. Thus, the electric pump is usually driven by a sensorless brushless motor (hereinafter referred to as a motor) that does not include mechanical contacting portions such as a brush and a commutator and that does not include a magnetic pole sensor, for example, a Hall element. The sensorless brushless motor detects a rotary position of a rotor by using an inductive voltage which is induced by a coil wound on a stator. The sensorless brushless motor switches the energization of each phase among three-phase windings in response to the information of magnetic pole position which is obtained by the detection of the rotary position of the rotor.
An electric fluid pump device disclosed in JP2004-166436A (hereinafter referred to as Patent reference 1) is provided with a brushless direct current (DC) motor which is driven by pulse width modulation (PWM) signals by an application of a sensorless driving circuit. The electric fluid pump device disclosed in Patent reference 1 includes a control unit that drives the brushless DC motor by current control and that switches the control of the motor from the current control to a rotational speed control when the rotational speed of a motor comes to be equal to or lower than a predetermined rotational speed of the motor. Accordingly, in a case where the rotational speed of the motor is greater than the predetermined rotational speed, the motor is controlled by the current control. When the load fluctuation of the motor occurs, the control unit switches the control of the motor from the current control to the rotational speed control in a case where a voltage applied on the motor decreases because of the current control and thereby the rotational speed of the motor decreases. The control unit switches the control of the motor from the current control to the rotational speed control in a case where the rotational speed of the motor comes to be equal to or lower than the predetermined rotational speed, so that the inductive voltage generated on the coil is prevented from decreasing, thereby inhibiting a loss of synchronism of the motor.
Further, a known electric pump supplying a necessary amount of an oil to a transmission even at a low temperature (mostly below zero Centigrade) is required. When a motor control is performed by an application of a voltage which is equivalent to a voltage applied to a motor when the oil is in a normal temperature, because the viscosity of the oil is increased at the low temperature, a rotary speed of the motor decreases because of the increase of a load applied to the motor in accordance with the increase of the viscosity of the oil. Because a discharge amount of the oil from the electric pump is insufficient, the required amount of oil cannot be supplied to the transmission. Further, because an inductive voltage generated on a coil decreases in accordance with the decrease of a rotary speed of the motor, the motor is stepped out when the rotary position of a rotor is not detected. In order to set the rotary speed of the motor equal to or higher than a predetermined amount at the low temperature by an application of the voltage on the motor, the voltage which is an equivalent to the voltage applied on the motor at a normal temperature, the upsizing of the motor is required to generate a large amount of a torque, thereby the electric pump using a sensorless brushless motor is inefficient in terms of space, weight and cost of the motor.
A need thus exists for an electric pump which is not susceptible to the drawback mentioned above.